THERMAL ENERGY STORAGE THROUGH MELTING OF A COMMERCIAL PHASE-CHANGE MATERIAL IN A HORIZONTAL CYLINDRICAL ANNULUS

A transient two-dimensional numerical model was developed to investigate the melting characteristics of an impure phase-change material (PCM) embedded between two concentric circular horizontal cylinders. The modeled transport equations were suitably nondimensionalized and were solved numerically in their primitive variables form on a staggered grid arrangement employing a controlvolume finite difference method. The selected PCM melts over a temperature range. To easily account for the latter aspect in the model, an enthalpy-porosity-based fixed grid scheme was used to solve the convection-diffusion mushy region phase-change problem. The inner cylindrical tube was heated to a constant temperature by a heat transfer fluid while the outer tube was insulated. Timewise evolutions of the temperature distributions are presented. Various quantities such as the average Nusselt number over the inner tube surface, the total melt fraction, and the total cumulative stored energy, all as a function of the melting time, are reported for three inner wall temperatures and for an initially saturated solid PCM as well as for a subcooled condition of 10 degrees C of the PCM. The predicted results show that the melting rate increases rapidly up to the melting time of about 41.18 min. After this time the melting rate increases but at a considerably slower rate. The storage of thermal energy increases with the increase of the inner wall temperature and initial temperature of the solid PCM. The energy charged is greatly influenced by the change of the inner tube wall temperature compared to the change of the initial solid PCM temperature.